Hey there!
D-Day was an extremely if not pivotal event that occurred during WW2. Hitler's principal goal during this war was to conquer as much land as possible as he wanted to be extremely powerful. After having taken over a ton of Europe, he wanted to invade France. However, the Allies (countries against the Nazis) made a surprise landing on the beaches of Normandy in France on D-Day, and there was a very deadly and treacherous battle that occurred, but the Germans lost, so the U.S., U.K., Soviet Union, and others were able to push the Nazis back to Germany due to tremendous amount of control and power that was gained by capturing Normandy and the other French beaches, which basically ended Hitler's reign of terror.
Have a wonderful day! :D